Field of the Invention
The invention relates to an object detection system for an inductive power transfer system, in particular for transferring power to a vehicle on a surface of a route. Further, the invention relates to a method for detecting foreign objects within an inductive power transfer system. Further, the invention relates to an inductive power transfer system and a vehicle.
Description of Related Art
While travelling on a route vehicles require energy for driving (i.e. propulsion) and for auxiliary equipment which does not produce propulsion of the vehicle. Such auxiliary equipment includes, for example, lighting systems, heating and/or air-conditioning systems, ventilation and passenger information systems. Not only track-bound vehicles (such as trams), but also road automobiles can be operated using electric energy. If continuous electric contact between the travelling vehicle and an electric rail or wire along the route is not desired, electric energy can be either withdrawn from an on-board energy storage or can be received by induction from an arrangement of electric lines of the route.
The transfer of electric energy to the vehicle by induction forms a background of the invention. A route-sided conductor arrangement (primary winding structure) of a primary unit of the system for inductive power transfer produces an electromagnetic field. The field is received by a secondary winding structure integrated into a receiving device on board of the vehicle so that the field produces an electric voltage by induction. The transferred energy may be used for propulsion of the vehicle and/or for other purposes such as providing the auxiliary equipment of the vehicle with energy.
A problem in such devices is that it is generally not possible to mechanically prevent foreign objects, in particular objects made of metal, from being placed into proximity of the primary unit of an inductive power transfer system, in particular from being placed within the air gap between the primary and the secondary winding structure. Such foreign objects may e.g. comprise a coin, a can, a key, a tool and other objects. The varying magnetic field generated by the primary winding structure may induce current in the foreign objects made of metal and in other objects or fluids. Such currents may cause power losses and heating of the object. In particular metal objects which are placed in a charging space between a route-sided charging pad which comprises the primary winding structure and the receiving device can heat up during inductive power transfer. Heating of the foreign objects may be dangerous for e.g. persons trying to touch and remove the foreign object and/or may damage the surface the foreign object is placed on or parts of the primary unit. Also, a heated object can cause fire. It is, for instance, possible that metallized paper may catch fire. Such metallized paper or other light packaging waste may e.g. be blown into the space between the primary and the secondary winding structure, e.g. due to weather conditions.
It is desirable to have an inductive power transfer system for private or public transport vehicle which is reliable and safe under all foreseeable conditions. E.g., if a person or a child places a metal object in said charging space, it is desirable to automatically turn off or derate the inductive transfer system.
WO 2013/036947 A2 discloses a foreign object debris detection system for a wireless energy transfer system comprising at least one magnetic field sensor and at least one readout circuit to measuring electrical parameters of the at least one magnetic field sensor. The at least one magnetic field sensor is positioned within the magnetic field of the wireless energy transfer system.
GB 2508923 discloses a safety system for an inductive power transfer system for transferring power to a vehicle on a surface of a route. The document discloses that a detection winding can be part of a LC oscillating circuit.
US 2008/0054905 A1 discloses metal detectors including a sense coil coupled to an analog to digital converter that produces a numeric representation of an electrical signal associated with the conductive object situated in an active region of a sense coil. The document describes that an electric signal in an alternating current part of a LC oscillating circuit is used to detect a foreign object.
U.S. Pat. No. 5,029,300 discloses a sensor which comprises an oscillating circuit including an LC resonant circuit, wherein a detection signal is prohibited upon arrival of an external radio wave.
The postpublished WO 2014/063159 A2 disclosure features apparatus, methods, and systems for wireless power transfer that include a power source featuring at least one resonator, a power receiver featuring at least one resonator, a first detector featuring one or more loops of conductive material and configured to generate an electrical signal based on a magnetic field between the power source and the power receiver, a second detector featuring conductive material, and control electronics coupled to the first and second detectors, where during operation, the control electronics are configured to measure the electrical signal of the first detector and compare the measured electrical signal of the first detector to baseline electrical information for the first detector to determine information about whether debris is positioned between the power source and the power receiver.
JP2014-39369 discloses a transmission device which comprises: a primary coil for transmitting power in a non-contact state with a power receiving part including a secondary coil provided in a vehicle; at least one detection coil provided around the primary coil; a resonance circuit connected to the detection coils; and processing sections connected to the resonance circuit to determine whether an object is detected on the basis of output from the resonance circuit. The detection coils are formed by winding a detection coil wire so as to surround the periphery of a detection axis, and the detection axis is arranged to be inclined from a vertical direction.